2 Answers
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Webapp2 is part of the Python 2.7 runtime but it's also a library compatible with Python 2.5 that you can download and use in your project like many other libraries.
Indeed, as noted in the documentation, Webapp2 can be used outside of GAE independently of the App Engine SDK.

To use it in your GAE Python 2.5 project, you don't need any additional download because Webapp2 is shipped with the GAE SDK and can be imported independently of the Runtime adopted *.

Here is a trivial example on how to use Webapp2 in the old Python 2.5 runtime: